1. Field of the Invention
This invention relates to a packaging material particularly suitable for packaging various photographic photosensitive materials such as X-ray film, lithfilm, sheet film, various photographic papers, presensitized plates and photosensitive resin film.
2. Description of the Prior Art
In general, the packaging materials for products of high technology such as photographic photosensitive materials are required to satisfy various properties such as gas barrier, moistureproofness, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, side welding (cut-sealability), hot tack properties (hot-sealability) and sealability with other materials, antistatic property, slipping character, low dusting characteristics and resistance to curling, as well as light-shielding ability to shield light completely.
Heretofore, various proposals have been made for satisfying the above properties.
The inventor has conducted investigations for improving the packaging materials for photosensitive materials, and for example, he has disclosed a packaging material wherein the physical strength is improved by combining two uniaxially stretched films (U.S. Pat. No. 4,331,725). The inventor has also disclosed a laminated film, composed of a foamed sheet and two uniaxially molecularly oriented thermoplastic resin films adhered on the both sides of the foamed sheet by melt adhesion directly or through an extrusion laminating adhesive layer so that respective molecular orientation axes cross each other at an angle of more than 30 degrees, of which the thickness is reduced to 40 to 85% of the total thickness by pressing (U.S. Pat. No. 4,565,733). The laminated film has a great impact puncture strength and Gelbo test strength, and it is excellent as a packaging material for heavy products.
However, the packaging material disclosed in U.S. Pat. No. 4,331,725 was inferior in heat sealing properties and was liable to be broken due to insufficient impact puncture strength and Gelbo test strength. The packaging material disclosed U.S. Pat. No. 4,565,733 was thick, and insufficient in antistatic properties as the packaging material for high-sensitive photographic photosensitive materials. It was also inferior in heat sealing properties in view of securing light-shielding, moistureproofness and the like and in bag-making ability for automatic bag-making machine, and it was expensive. In both packaging materials, trimming loss was also a problem in the lamination.
Therefore, the inventor has conducted an investigation to develop a light-shielding thick packaging material which is coextruded multilayer inflation film using L-LDPE resin blended with carbon black which is excellent in heat sealing properties, tear strength and impact puncture strength. The packaging material has a great physical strength and is only slightly curled. However, when the thickness was more than 120 .mu.m, melt fracture occurred.